Organic light-emitting diodes (OLEDs) are known in the art. For example, Hung et al. (U.S. Pat. No. 5,776,623) also discloses an electroluminescent device wherein a 15 nm-thick CuPc layer is used as a hole injection layer (HIL), a 60 nm-thick NPB layer is used as a hole transport layer (HTL), a 75 nm-thick Alq3 layer is used as an electron transport layer (ETL). A 0.5 nm-thick lithium fluoride layer is also deposited on the Alq3 layer. The lithium fluoride layer can be replaced by a magnesium fluoride, a calcium fluoride, a lithium oxide or a magnesium oxide layer.
Kido et al. (U.S. Pat. No. 6,013,384) discloses, as shown in FIG. 1a, an organic electroluminescent device 10 wherein the optoelectronic sub-structure consists of a hole transport layer (HTL) 13, a luminescent layer 14 and a metal-doped organic compound layer 15 disposed between an anode layer 12 and a cathode layer 16. The device is fabricated on a substrate 11.
Weaver et al (U.S. Publication No. 2004/0032206 A1) discloses another OLED including an alkali metal compound layer. As shown in FIG. 1b, the OLED 20 is fabricated on a plastic substrate 21 pre-coated with an ITO anode 22. The cathode consists of two layers: a metal oxide layer 28 deposited over a layer 27 of Mg or Mg alloy, such as an alloy of Mg and Ag. The alkali metal compound layer 26 can be made of alkali halides or alkali oxides such as LiF and Li2O. The organic layers include an HTL layer 23, an emissive layer (EML) 24 and an electron transport layer (ETL) 25.
Raychaudhuri et al. (U.S. Pat. No. 6,551,725 B2) discloses an OLED 30 wherein a buffer structure is disposed between the organic layer and the cathode. As shown in FIG. 1c, the buffer structure consists of two layers, a first layer 37 containing an alkali halide is provided over the electron transport layer (ETL) 36, and a second buffer layer 38 containing a metal or metal alloy having a work function between 2.0 and 4.0 eV is provided over the first buffer layer 37. In addition, a hole injection layer (HIL) 33 is provided between the anode 32 and the organic layers. The hole injection layer can be made of a porphorinic or phthalocyanine compound. The hole injection layer can also be made of a fluorinated polymer CFx, where x is 1 or 2. The hole transport layer (HTL) 34 can be made of various classes of aromatic amines. The emissive layer (EML) 35 provides the function of light emission produced as a result of recombination of holes and electrons in the layer. The cathode layer 39 is made by sputter deposition to provide increased conductivity and reflectivity of the electron injection layer of the device.
A generalized OLED structure is shown in FIG. 2. The hole injection and transport layers together can be treated as a hole source. The electron injection and transport layers together can be treated as an electronic source. One or both the electron source and the hole source can be made of organic or inorganic materials. The emissive layer is made of an organic host material doped with a fluorescent or phosphorescent dopant. In general, the electrical conductivity of prior art interlayers is lower than 10−8 S/cm (S=Ω−1), limiting the light-emitting efficiency of the device.
It is known that a PIN diode is a photoelectric device with a large, neutrally doped intrinsic region sandwiched between p-doped and n-doped semiconducting regions. The doping in the p-doped and n-doped regions significantly increases the electrical conductivity of the semiconductor material and the efficiency of the device.
It would be desirable and advantageous to improve the device efficiency of an OLED by changing the electrical conductivity distribution in various layers in the OLED.